If you want MM stats, I submit the nycaloth for consideration. It's an excellent ambusher: darkness, invisibility, blindsight, teleportation, ongoing bleed damage, damage resistances, etc. This nasty piece of work has everything you want. At CR9 it will be a pushover for a party of that level in a straight fight. It will need to creep around, weaken the party and use the dungeon to its advantage. I'd also consider giving it trollish regeneration to make it start each encounter fresh--and severed alien limbs attacking the PCs is too good to pass up! The alien face-hugger concept works better in a movie than it will do in an adventure. The slaadi were obviously inspired by Alien, but are more suitable to a weird dimensional invasion storyline than a horror scenario (the tadpoles are weak and easy to kill and the gestation period is three months making it a cruel gotcha for players rather than anything important in the time frame of the adventure itself).
In any case, you should reskin whatever monster you use. Make it more 'alien', more weird. The suspense in Alien comes from the characters not having any rational template for what they are dealing with. Make the appearance more nightmarish--a disgusting mishmash of tentacles, mouths, carapaces, spines, dripping slime, etc. Give it a slight tell-tale sign that its in the general area--such as a faint odor or buzzing sound. Too faint for the PCs to pin-point its location but enough to put them on edge after they learn its significance.
Alien also has the classic slasher film trope of dispatching the characters one-by-one. Unless you want a potential TPK on your hands, you'll need to forgo this aspect. The PCs will not willingly split up or put themselves in danger. They will NOT cooperate to serve themselves up on a platter. To keep the suspense up, you should add allied NPCs to the adventure for the monster to dispatch. Perhaps another adventuring team, soldier's from the king or a group hired by the wizard's order. Play up their off-screen deaths and relish your descriptions of the gory aftermath that the PCs encounter.
Also, consider the dungeon environment. There should be numerous other challenges for the PCs to deal with. Perhaps a self-destruction mechanism that starts when the door is unsealed placing the adventure on a ticking clock. Traps are essentially, especially those that split up the party allowing the monster to attack: falling portcullises, pit traps that lead to chutes, gaps that need to be crossed, crawl-spaces, etc. Also the wizard problem found other bizarre creatures and objects so you have to those in there. Could be a very memorable adventure.
